README-dynapi.txt - external/github.com/libsdl-org/SDL - Git at Google

 ================================================================================
 Dynamic API
 ================================================================================
 Originally posted by Ryan at https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U

 Background:

 - The Steam Runtime has (at least in theory) a really kick-ass build of SDL2,
   but developers are shipping their own SDL2 with individual Steam games.
   These games might stop getting updates, but a newer SDL2 might be needed later.
   Certainly we'll always be fixing bugs in SDL, even if a new video target isn't
   ever needed, and these fixes won't make it to a game shipping its own SDL.
 - Even if we replace the SDL2 in those games with a compatible one, that is to
   say, edit a developer's Steam depot (yuck!), there are developers that are
   statically linking SDL2 that we can't do this for. We can't even force the
   dynamic loader to ignore their SDL2 in this case, of course.
 - If you don't ship an SDL2 with the game in some form, people that disabled the
   Steam Runtime, or just tried to run the game from the command line instead of
   Steam might find themselves unable to run the game, due to a missing dependency.
 - If you want to ship on non-Steam platforms like GOG or Humble Bundle, or target
   generic Linux boxes that may or may not have SDL2 installed, you have to ship
   the library or risk a total failure to launch. So now, you might have to have
   a non-Steam build plus a Steam build (that is, one with and one without SDL2
   included), which is inconvenient if you could have had one universal build
   that works everywhere.
 - We like the zlib license, but the biggest complaint from the open source
   community about the license change is the static linking. The LGPL forced this
   as a legal, not technical issue, but zlib doesn't care. Even those that aren't
   concerned about the GNU freedoms found themselves solving the same problems:
   swapping in a newer SDL to an older game often times can save the day.
   Static linking stops this dead.

 So here's what we did:

 SDL now has, internally, a table of function pointers. So, this is what SDL_Init
 now looks like:

     UInt32 SDL_Init(Uint32 flags)
     {
         return jump_table.SDL_Init(flags);
     }

 Except that is all done with a bunch of macro magic so we don't have to maintain
 every one of these.

 What is jump_table.SDL_init()? Eventually, that's a function pointer of the real
 SDL_Init() that you've been calling all this time. But at startup, it looks more
 like this:

     Uint32 SDL_Init_DEFAULT(Uint32 flags)
     {
         SDL_InitDynamicAPI();
         return jump_table.SDL_Init(flags);
     }

 SDL_InitDynamicAPI() fills in jump_table with all the actual SDL function
 pointers, which means that this _DEFAULT function never gets called again.
 First call to any SDL function sets the whole thing up.

 So you might be asking, what was the value in that? Isn't this what the operating
 system's dynamic loader was supposed to do for us? Yes, but now we've got this
 level of indirection, we can do things like this:

     export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0
     ./MyGameThatIsStaticallyLinkedToSDL2

 And now, this game that is staticallly linked to SDL, can still be overridden
 with a newer, or better, SDL. The statically linked one will only be used as
 far as calling into the jump table in this case. But in cases where no override
 is desired, the statically linked version will provide its own jump table,
 and everyone is happy.

 So now:
 - Developers can statically link SDL, and users can still replace it.
   (We'd still rather you ship a shared library, though!)
 - Developers can ship an SDL with their game, Valve can override it for, say,
   new features on SteamOS, or distros can override it for their own needs,
   but it'll also just work in the default case.
 - Developers can ship the same package to everyone (Humble Bundle, GOG, etc),
   and it'll do the right thing.
 - End users (and Valve) can update a game's SDL in almost any case,
   to keep abandoned games running on newer platforms.
 - Everyone develops with SDL exactly as they have been doing all along.
   Same headers, same ABI. Just get the latest version to enable this magic.


 A little more about SDL_InitDynamicAPI():

 Internally, InitAPI does some locking to make sure everything waits until a
 single thread initializes everything (although even SDL_CreateThread() goes
 through here before spinning a thread, too), and then decides if it should use
 an external SDL library. If not, it sets up the jump table using the current
 SDL's function pointers (which might be statically linked into a program, or in
 a shared library of its own). If so, it loads that library and looks for and
 calls a single function:

     SInt32 SDL_DYNAPI_entry(Uint32 version, void *table, Uint32 tablesize);

 That function takes a version number (more on that in a moment), the address of
 the jump table, and the size, in bytes, of the table.
 Now, we've got policy here: this table's layout never changes; new stuff gets
 added to the end. Therefore SDL_DYNAPI_entry() knows that it can provide all
 the needed functions if tablesize <= sizeof its own jump table. If tablesize is
 bigger (say, SDL 2.0.4 is trying to load SDL 2.0.3), then we know to abort, but
 if it's smaller, we know we can provide the entire API that the caller needs.

 The version variable is a failsafe switch.
 Right now it's always 1. This number changes when there are major API changes
 (so we know if the tablesize might be smaller, or entries in it have changed).
 Right now SDL_DYNAPI_entry gives up if the version doesn't match, but it's not
 inconceivable to have a small dispatch library that only supplies this one
 function and loads different, otherwise-incompatible SDL libraries and has the
 right one initialize the jump table based on the version. For something that
 must generically catch lots of different versions of SDL over time, like the
 Steam Client, this isn't a bad option.

 Finally, I'm sure some people are reading this and thinking
 "I don't want that overhead in my project!"
 To which I would point out that the extra function call through the jump table
 probably wouldn't even show up in a profile, but lucky you: this can all be
 disabled. You can build SDL without this if you absolutely must, but we would
 encourage you not to do that. However, on heavily locked down platforms like
 iOS, or maybe when debugging,  it makes sense to disable it. The way this is
 designed in SDL, you just have to change one #define, and the entire system
 vaporizes out, and SDL functions exactly like it always did. Most of it is
 macro magic, so the system is contained to one C file and a few headers.
 However, this is on by default and you have to edit a header file to turn it
 off. Our hopes is that if we make it easy to disable, but not too easy,
 everyone will ultimately be able to get what they want, but we've gently
 nudged everyone towards what we think is the best solution.
	================================================================================
	Dynamic API
	================================================================================
	Originally posted by Ryan at https://plus.google.com/103391075724026391227/posts/TB8UfnDYu4U

	Background:

	- The Steam Runtime has (at least in theory) a really kick-ass build of SDL2,
	but developers are shipping their own SDL2 with individual Steam games.
	These games might stop getting updates, but a newer SDL2 might be needed later.
	Certainly we'll always be fixing bugs in SDL, even if a new video target isn't
	ever needed, and these fixes won't make it to a game shipping its own SDL.
	- Even if we replace the SDL2 in those games with a compatible one, that is to
	say, edit a developer's Steam depot (yuck!), there are developers that are
	statically linking SDL2 that we can't do this for. We can't even force the
	dynamic loader to ignore their SDL2 in this case, of course.
	- If you don't ship an SDL2 with the game in some form, people that disabled the
	Steam Runtime, or just tried to run the game from the command line instead of
	Steam might find themselves unable to run the game, due to a missing dependency.
	- If you want to ship on non-Steam platforms like GOG or Humble Bundle, or target
	generic Linux boxes that may or may not have SDL2 installed, you have to ship
	the library or risk a total failure to launch. So now, you might have to have
	a non-Steam build plus a Steam build (that is, one with and one without SDL2
	included), which is inconvenient if you could have had one universal build
	that works everywhere.
	- We like the zlib license, but the biggest complaint from the open source
	community about the license change is the static linking. The LGPL forced this
	as a legal, not technical issue, but zlib doesn't care. Even those that aren't
	concerned about the GNU freedoms found themselves solving the same problems:
	swapping in a newer SDL to an older game often times can save the day.
	Static linking stops this dead.

	So here's what we did:

	SDL now has, internally, a table of function pointers. So, this is what SDL_Init
	now looks like:

	UInt32 SDL_Init(Uint32 flags)
	{
	return jump_table.SDL_Init(flags);
	}

	Except that is all done with a bunch of macro magic so we don't have to maintain
	every one of these.

	What is jump_table.SDL_init()? Eventually, that's a function pointer of the real
	SDL_Init() that you've been calling all this time. But at startup, it looks more
	like this:

	Uint32 SDL_Init_DEFAULT(Uint32 flags)
	{
	SDL_InitDynamicAPI();
	return jump_table.SDL_Init(flags);
	}

	SDL_InitDynamicAPI() fills in jump_table with all the actual SDL function
	pointers, which means that this _DEFAULT function never gets called again.
	First call to any SDL function sets the whole thing up.

	So you might be asking, what was the value in that? Isn't this what the operating
	system's dynamic loader was supposed to do for us? Yes, but now we've got this
	level of indirection, we can do things like this:

	export SDL_DYNAMIC_API=/my/actual/libSDL-2.0.so.0
	./MyGameThatIsStaticallyLinkedToSDL2

	And now, this game that is staticallly linked to SDL, can still be overridden
	with a newer, or better, SDL. The statically linked one will only be used as
	far as calling into the jump table in this case. But in cases where no override
	is desired, the statically linked version will provide its own jump table,
	and everyone is happy.

	So now:
	- Developers can statically link SDL, and users can still replace it.
	(We'd still rather you ship a shared library, though!)
	- Developers can ship an SDL with their game, Valve can override it for, say,
	new features on SteamOS, or distros can override it for their own needs,
	but it'll also just work in the default case.
	- Developers can ship the same package to everyone (Humble Bundle, GOG, etc),
	and it'll do the right thing.
	- End users (and Valve) can update a game's SDL in almost any case,
	to keep abandoned games running on newer platforms.
	- Everyone develops with SDL exactly as they have been doing all along.
	Same headers, same ABI. Just get the latest version to enable this magic.


	A little more about SDL_InitDynamicAPI():

	Internally, InitAPI does some locking to make sure everything waits until a
	single thread initializes everything (although even SDL_CreateThread() goes
	through here before spinning a thread, too), and then decides if it should use
	an external SDL library. If not, it sets up the jump table using the current
	SDL's function pointers (which might be statically linked into a program, or in
	a shared library of its own). If so, it loads that library and looks for and
	calls a single function:

	SInt32 SDL_DYNAPI_entry(Uint32 version, void *table, Uint32 tablesize);

	That function takes a version number (more on that in a moment), the address of
	the jump table, and the size, in bytes, of the table.
	Now, we've got policy here: this table's layout never changes; new stuff gets
	added to the end. Therefore SDL_DYNAPI_entry() knows that it can provide all
	the needed functions if tablesize <= sizeof its own jump table. If tablesize is
	bigger (say, SDL 2.0.4 is trying to load SDL 2.0.3), then we know to abort, but
	if it's smaller, we know we can provide the entire API that the caller needs.

	The version variable is a failsafe switch.
	Right now it's always 1. This number changes when there are major API changes
	(so we know if the tablesize might be smaller, or entries in it have changed).
	Right now SDL_DYNAPI_entry gives up if the version doesn't match, but it's not
	inconceivable to have a small dispatch library that only supplies this one
	function and loads different, otherwise-incompatible SDL libraries and has the
	right one initialize the jump table based on the version. For something that
	must generically catch lots of different versions of SDL over time, like the
	Steam Client, this isn't a bad option.

	Finally, I'm sure some people are reading this and thinking
	"I don't want that overhead in my project!"
	To which I would point out that the extra function call through the jump table
	probably wouldn't even show up in a profile, but lucky you: this can all be
	disabled. You can build SDL without this if you absolutely must, but we would
	encourage you not to do that. However, on heavily locked down platforms like
	iOS, or maybe when debugging, it makes sense to disable it. The way this is
	designed in SDL, you just have to change one #define, and the entire system
	vaporizes out, and SDL functions exactly like it always did. Most of it is
	macro magic, so the system is contained to one C file and a few headers.
	However, this is on by default and you have to edit a header file to turn it
	off. Our hopes is that if we make it easy to disable, but not too easy,
	everyone will ultimately be able to get what they want, but we've gently
	nudged everyone towards what we think is the best solution.